Paper products, e.g., tissues, have conventionally been manufactured by forming a wet paper web on a fabric-carrying sheet, which then carries the paper web through a pressing section to remove the excess water from the web. After pressing the web to remove excess water, the paper web would then be fed to a separate drying section to fully remove the remaining moisture from the web. This step of pressing the web, however, reduces bulk and absorbency. Therefore, as opposed to leaving the web as a flat sheet on a single plane, rotatable air-heated drum dryers over which the web traveled were utilized in combination with an imprinting fabric sheet. Dryer hoods and air supply ducts are widely used in connection with these rotary drums, wherein pressurized drying air is introduced into the roll or at various points in the hood to contact one exposed surface of the wet web as it progresses around the dryer with the exit path for the air being positioned on the other side of the rotary drum. This process is known as through air drying (TAD).
U.S. Pat. No. 3,303,576 issued to Sisson discloses one such drying assembly in which a moving stream of pressurized drying air is circulated about a paper web traveling about the periphery of a rotatable roll having apertures formed therein. Sisson utilizes a system where the hot drying air travels from the inside of the rotatable roll to the outside through the apertures, while the web travels about the outer surface of the roll. With such rotatable rolls usually being composed of metal, this inside-to-outside type drying requires smaller diameter rolls because a larger diameter results in the web and fabric sheet lifting away from the roll surface at the air flows and pressures of commercial interest. Also, the drum metal is exposed to the full air temperature, which has adverse consequences with respect to maintenance and drum life.
U.S. Pat. No. 3,432,936 to Cole et al. avoids the problems with inside-to-outside drying by employing a configuration, which moves drying air from the exterior of a rotatable roll through the paper web and into the interior of the rotatable roll, otherwise known as outside-to-inside drying. Web and fabric sheet lifting do not impose airflow restrictions in outside-to-inside drying, because the air is blowing them onto the roll surface. Also, this configuration positions the metal rotatable roll on the cool side of the paper web, which allows for improved maintenance and drum life. However, when two or more of these rotatable rolls employing outside-to-inside drying are used, at least two carrying rolls must contact the paper web. Whenever wet paper webs contact carrying rolls, machine runability problems as well as product quality problems may be encountered.
One of the most important shortcomings associated with the above-described paper drying machine is that the paper web must come into contact with a carrying roll whenever more than one rotatable drying roll is employed. Further, machines that have one roll have limited drying capacity and are of limited commercial interest for that reason. Moreover, prior art machines employing multiple drying rolls have the drying air traveling in the same direction in each of the drying rolls used in such a machine. However, the most efficient use of space in these machines would be to use a combination of inside-to-outside drying rolls with outside-to-inside drying rolls. U.S. Pat. No. 1,718,573 issued to Millspaugh discloses a paper making machine which discloses removing moisture from a paper web in an outside-to-inside fashion using a suction roll followed by an inside-to-outside removal of moisture by forcing steam through the paper web as it passes over a blower roll. It should be noted, however, that the device disclosed by Millspaugh utilizes steam with its blower roll.
Because of the water content of steam, the medium cannot be used to dry a sheet to the required solids content. Furthermore, it is economically more efficient and proficient to employ heated air rather than a combination of air and steam when using a blowing device to dry a wet paper web.
U.S. Pat. No. 5,636,452 to Thorp et al. discloses a drying apparatus having two TAD rolls with one TAD employing inside-to-outside drying air and the other TAD roll employing outside-to-inside drying air. Thorp et al. is limiting in that its drying capacity is not sufficient to reach world class speeds found in present drying machines. The surface drying area of the two TAD rolls of Thorp et al. is less than that found in typical new TAD drying machines. Additionally, the configuration of Thorp et al. produces limited wrap of the paper web around its outside-to-inside drying drum, providing less drying surface area. In order to increase wrap, without the use of an outside roll that would touch the paper web, both TAD rolls of Thorp et al. are oriented at an angle. This leads to the need to have the exhaust hoods positioned at an angle as well, resulting in a complex and expensive design, especially when retrofitting existing machines.
It is an object of the present invention to provide a paper drying machine that reduces or wholly overcomes some or all of the difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of preferred embodiments.
The principles of the invention may be used to advantage to provide a drying machine having improved drying capacity. Such a construction advantageously permits all hoods of the dryer to be positioned in the customary vertical position, thereby significantly reducing the time and costs associated with retrofitting existing machines. This also provides greater flexibility, and takes advantage of vertical space, which is typically more available when retrofitting existing drying machines. Additionally, the present invention may provide greater wrap around its drying drums in certain embodiments. Further, the addition of a third TAD roll significantly increases the drying capacity of the machine.
In accordance with a first aspect, a paper drying apparatus to dry a paper web carried on a fabric sheet includes a first rotatable drum to carry a paper web. A second rotatable drum to carry the paper web is positioned downstream of the first rotatable drum with respect to the paper web carried by the first and second rotatable drums. A third rotatable drum to carry the paper web is positioned downstream of the second rotatable drum with respect to the paper web carried by the first, second, and third rotatable drums. A first air supply supplies air through the first rotatable drum in a first direction to dry the paper web. A second air supply supplies air through the second rotatable drum in a second direction opposite to the first direction to dry the paper web. A third air supply supplies air through the third rotatable drum in the first direction to dry the paper web. The first direction is from the inside of a respective drum toward the outside of the respective drum, or from the outside of the respective drum toward the inside of the respective drum.
In accordance with another aspect, a method for drying a paper web being carried on a fabric sheet includes the steps of guiding a paper web toward a first rotatable drum; passing the paper web over the first rotatable drum; forcing air through the paper web in a first direction, with the first direction being one of a direction from the inside of the first rotatable drum toward the outside of the first rotatable drum and the outside of the first rotatable drum toward the inside of the first rotatable drum; guiding the paper web from the first rotatable drum toward a second rotatable drum; passing the paper web over the second rotatable drum; forcing air through the paper web about the second rotatable drum in a second direction, the second direction being opposite to the first direction; guiding the paper web from the second rotatable drum toward a third rotatable drum; passing the paper web over the third rotatable drum; and forcing air through the paper web about the third rotatable drum in the first direction.
From the foregoing disclosure, it will be readily apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this area of technology, that the present invention provides a significant advance. Preferred embodiments of the paper drying machine of the present invention can provide improved drying capacity, while reducing the cost of retrofitting existing paper drying machines. These and additional features and advantages of the invention disclosed here will be further understood from the following detailed disclosure of preferred embodiments.